Jadon and Anias were born conjoined at the head in Chicago in September 2015. The boys are technically "craniopagus twins"—a phenomenon that occurs once in every 2.5 million births.

Around 80 per cent of craniopagus twins die of medical complications by the age of two if not separated, according to studied cited by CNN.

So to ensure the best chance of survival, the boys and their parents traveled to Montefiore Hospital in New York. There, the twins underwent a surgery costing $2.5 million to separate them.

Jadon and Anias were born in September 2015, conjoined at the head. (Facebook: Nicole McDonald)

The risky, extremely rare operation began on Thursday October 13th, and took 27 hours to complete.

Both boys survived. However, separation means one or both may suffer developmental complications and Anias' recovery has been slower than Jadon's, meaning his parents have not been able to hold him yet.

The McDonalds, with their older son Aza, when Nicole was pregnant with the twins. (Photo: GoFundMe)

McDonald and her husband Christian have made it clear they will love their boys regardless of the eventual outcome.

"For over 13 months, I've dreamed of this moment," McDonald posted on Facebook.

"I looked down at Jadon's angelic face and saw him in a way I'd never seen him before," she added, describing it as: "One of the most profound moments of my life."

Anais in hospital. (Feature: Nicole McDonald)

The boys' survival to thirteen months is remarkable, given that around 40 percent of craniopagus twins are stillborn. Of those that survive, a third die within 24 hours of birth, CNN reports.

A GoFundMe page to help the family with the cost of the expensive surgery has raised almost $283,000 so far.